The invention relates generally to parallel plate, radial lines and more particularly, to radial lines having high isolation.
Conventional power divider/combiners use branching transmission line networks that start from a single input port and branch out to N output ports (where N is the number of such ports) and vice versa for a combiner. Such networks are commonly known as corporate feeds. A corporate feed that uses simple three port T-junctions at each branch point is known as a reactive feed. As is well known, a three port junction is not impedance matched looking into all ports, (see Montgomery, Purcell and Dicke, MIT Rad. Lab. Series Vol. 8, Principles of Microwave Circuits, Chapter 9), hence, spurious reflections from any source such as at any other junction, connectors, bends etc. within the corporate feed or from any device at any of the outputs can cause large errors in the output amplitudes and phases and can cause resonances within the feed network. As a result, they can cause undesirable mutual coupling between the output devices, such as amplifiers, to result in spurious reflections or oscillations with a loss of efficiency and in some cases, high power breakdown. If each simple three port T-junction were replaced by a matched four port hybrid such as a magic-T or quadrature hybrid, these problems would be greatly alleviated because the spurious reflections are absorbed in the matched loads in the fourth port of the hybrid junction (see R. C. Johnson and H. Jasik, Antenna Engineering Handbook, Second Edition, pp. 20-55 through 20-56 and pg. 40-18).
A corporate feed using the above-described hybrid arrangement is typically quite complex, large, and costly because it contains on the order of N-1 hybrids, N-1 terminating loads, 2(N-1) bends and interconnecting transmission lines. It is also relatively lossy because, for cost purposes, the corporate feed is usually designed in stripline or microstrip which are very lossy compared to waveguide. As the number of power divisions increases, waveguide also becomes a relatively lossy technique. Also, stripline and microstrip have not been able to handle high peak or high average powers.
The radial line power combiner is used for combining the outputs of a plurality of circumferentially mounted power sources in a single combining structure. Likewise, it is usable for dividing an input signal into a plurality of output signals in a single structure. By using two radial lines, one functioning as a power divider and the other as a power combiner, a high power transmitter may be formed by coupling a plurality of individual power amplifying devices to the circumferences of both radial lines. However, in prior radial line techniques, the failure of an amplifier or amplifiers or the mismatching of a part of the radial line caused the generation of higher order modes with a resulting decrease in radial line efficiency and power output. Amplifiers such as injection locked impact avalanche and transit time (IMPATT) diodes are extremely sensitive to mismatches.
A prior technique used to suppress higher order modes in a radial line involves mounting resistors at the circumference of the radial line between the power sources. This technique is difficult to implement at the higher frequencies such as millimeter wave where the resistor size is small, thus making it difficult to handle. Also the use of a discrete resistor may limit the power handling capabilty of the radial line. It has been found that the isolation obtained by such an arrangement was generally not adequate for such sensitive amplifiers as injection locked IMPATT diodes.
Accordingly, it is an object of the invention to provide a radial line power divider/combiner which has the advantages of a radial line and which suppresses undesirable modes to a greater extent.
It is also an object of the invention to provide a radial line power divider/combiner which is able to handle relatively large power levels more efficiently.